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I am in the process of clearing out old random wires from my 100 year old house (think multiple layers, modern, knob and tube and everything in between). In the process of doing this, I tried to remove a knob and tube circuit that disappeared into an air duct. I turned off the breaker feeding it and cut the wires before they entered the duct. Why is the wiring being traced through an air duct? You'll have to ask someone from 100 years ago.

I then went to the panel to disconnect the wires completely from the breaker (leaving the loose in between pieces in the air return, as I don't want to take that apart). The hot wire is easy to identify being connected to a specific breaker. However, it enters the panel by itself, as a single cloth wrapped conductor not linked in any way to the neutral. The neutral is one of 30 other neutral wires entering the panel, and I can't easily see which one it is. Is it safe to leave the neutral wire that is buried in the air duct connected to the neutral bar, or should I really unearth it so I can fully remove it from its panel connection.

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    If you're doing this much work, why not go all out and redo all the wiring to bring everything up to modern standards? – Hari Ganti Apr 3 '17 at 20:39
  • If you know where this wire ends up, and you disconnected the conductor from the breaker, you can temporaily nut the hot and neutral together and then test the hot wire originally at the breaker to each neutral wire at the breaker. The neutral with 0 resistance is the one you need to remove. – maple_shaft Apr 3 '17 at 22:32
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    @maple_shaft, Wouldn't you have to temporarily disconnect each neutral in turn while you test? Otherwise, they're all just connected together on a common block are they not? – Octopus Apr 3 '17 at 23:07
  • @Octopus Yeah I guess that would be a pain huh? Cant really think of a way to for sure tell if you cant really see where the incoming wire is coming. – maple_shaft Apr 3 '17 at 23:16
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    US wiring is weird... in the UK we have this thing called a "cable", which contains a live wire, a neutral wire and an earth wire. It's easy to tell which neutral goes with which live - they're the ones in the same cable! – AndyT Apr 4 '17 at 11:23
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Chase down the matching neutral and remove it.

Otherwise it will drive you crazy, and be a loose end of wire unaccounted for. What's more, it could wind up still being in use by some other circuit which shouldn't be there. Part of the reason to fully explore an electrical system is to look for surprises.

Besides, since all wires must terminate in junction boxes, it will violate code and fail inspection.

I don't agree with my colleague that it's necessary to tear apart the ducting system, I feel unreachability is a defense. But to play that defense, you must positively identify both wires of that circuit, both where they enter and exit the ducting, and fully remove all of that.

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    When I sold real estate and worked with home inspectors we were always aware that there is no requirement to bring homes up to modern code except for major renovations. Of course, safety hazards would be of concern in a case like this. But I'm with you - it would drive me crazy until I removed everything from that duct. – SDsolar Apr 5 '18 at 0:19
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Wiring has no business in air ducts, so get it out of there

For safety reasons (burning wire insulation in air ducts smoking you out of your house is majorly no bueno), the NEC prohibits wiring in ductwork that isn't required by the HVAC system design in 300.22(B):

(B) Ducts Specifically Fabricated for Environmental Air. Equipment, devices, and the wiring methods specified in this section shall be permitted within such ducts only if necessary for the direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to facilitate maintenance and repair, enclosed gasketed-type luminaires shall be permitted.

Only wiring methods consisting of Type MI cable without an overall nonmetallic covering, Type MC cable employing a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering shall be installed in ducts specifically fabricated to transport environmental air. Flexible metal conduit shall be permitted, in lengths not to exceed 1.2 m (4 ft), to connect physically adjustable equipment and devices permitted to be in these fabricated ducts. The connectors used with flexible metal conduit shall effectively close any openings in the connection.

So, I'd get that ancient, stray wire out of the duct system and removed completely.

  • It's more likely that the wiring was there before someone decided to use the space between joists as a duct. Forced-air is newer than K&T and the previous system, gravity, needed fairly particular ducting. – Harper - Reinstate Monica Apr 3 '17 at 22:41
  • @Harper -- talk about a silly tinbender for an HVAC guy, then... – ThreePhaseEel Apr 3 '17 at 23:20
  • Yup, what you hear talking there is my aversion to crafts I'm not skilled at. Wire up a pad transformer, sure, actually pour the concrete pad, "yikes!" – Harper - Reinstate Monica Apr 3 '17 at 23:24
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    Thank you all (esp Harper and Three) for your guidance. I think you are right, and will figure out a strategy to unearth the neutral and expunge it once and for all. I agree about the chronology: K/T first, then duct. Living in a house like this is like being an archeologist, where when you scrape a layer you discover signs of previous civilizations. – Simon Apr 4 '17 at 4:56
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You can die from cutting the wrong neutral wire... (but it's not that easy)

There is something known as a 3-wire circuit that attempts to take advantage of AC's properties. Intended to let you use a smaller neutral wire than the two hot wires it supports. This is only done for hots sourced from different hot "legs" at the panel. This is supposed to have a single handle double breaker so that both hots get shut off at the same time.

Presuming a 220v service at panel supplying 110v home uses -> The plan is commonly that you have a TV in one room and a table lamp in the next room sharing the same neutral wire, but they each have their own hot leg from different sides of the supply transformer (down the street on a pole somewhere). If you watch TV the neutral has only that load to carry. If you go read a book then the neutral only carries the lamp. BUT if you leave the TV on and go read a book, the neutral carries NOTHING since the TV and lamp are now in series in a 220v circuit.

The problem-> Cutting the neutral between the connection to use points and the panel can leave the 220v lines available to possibly hurt you and neither TV or lamp will work without the other.

It would take a very good psychic to guess what may have been done properly or otherwise in a house that old, so I agree with earlier advice - chase it down. You certainly don't want your duct work becoming energized

  • An imbalanced load will also wreck havoc on the loads themselves because one load will get more voltage than it is meant to support if the neutral was cut. – ratchet freak Apr 4 '17 at 10:18
  • True enough. If that neutral is carrying a leakage current then cutting it could result in the whole duct becoming energized. – SDsolar Apr 5 '18 at 0:21

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